The present invention relates to machine tools, and more particularly to a jig grinder having automatic C-axis feedrate control to provide a constant feedrate for straight and arc grinding.
Jig grinders are used by machinists to accurately grind precision holes, radii, blends, and surfaces in metal or other materials. These machine tools are used in the manufacture of a wide variety of high precision products. For example, jig grinders are used by the automotive and aerospace industries for producing engine parts, gears, and the like. Although extreme precision is the primary goal of such applications, productivity is a main concern, particularly in keeping the cost of individual machine parts as reasonable as possible. Accordingly, recent advances in jig grinder design have concentrated on automating the grinding process in an effort to increase machine throughput while maintaining accuracy.
Various grinding techniques which can be accomplished with jig grinders are well known. One such technique is the use of "chop grinding". The chop grinding principle utilizes a rapid reciprocation of the grinding wheel in a direction along the wheel axis. It has been found that this technique grinds cool, providing more consistent surface finishes, geometry and size than other, more conventional grinding techniques.
Chop grinding can be used to machine holes and edges as well as radii. There are several different motions which occur simultaneously during a chop grinding process. One motion is that of the grinding wheel which rotates, typically at a high rate of speed. The grinding wheel, which is coupled to the spindle of the jig grinder, can be offset from the center axis of the spindle (along the machine's "U-axis") so that it will sweep an arc when the spindle is rotated. This motion is useful when grinding arc segments (i.e. curved surfaces) in a workpiece. Rotation of the spindle occurs along the "C-axis" of the jig grinder. As noted above, the grinding wheel will also be reciprocated up and down. Reciprocation of the grinding wheel occurs along the "Z'-axis" of the jig grinder. Further, the workpiece being machined is usually mounted to a worktable, which can be moved in straight paths described by the "X" and/or "Y" axes of the jig grinder.
In the past, jig grinders have been largely manually operated. In performing a chop grinding task, a machine operator would manually index the grinding wheel against the surface to be ground, and commence grinding with a rotating and reciprocating grinding wheel. As material was ground away from the surface of the workpiece, the machine operator would manually feed the grinding wheel toward the workpiece ("outfeed") until the required amount of material was ground off. Upon each outfeed operation, the grinding wheel would create a shower of sparks as it proceeded to strip material from the grinding surface. A machine operator would then wait until the sparks stopped, indicating that the grinding wheel had removed all of the stock up to the outfed position (or the grinding wheel had worn to a point where an outfeed adjustment was necessary). The operator would then manually repeat the cycle until the finished dimensions were reached.
Wipe grinding, which is a technique wherein the grinding wheel is not reciprocated, has also been done manually in the past. An operator would bring the grinding wheel to the surface to be ground, and manually index the grinding wheel toward the workpiece until the required amount of stock material had been removed. When wipe grinding, an operator would slowly and repetitively feed the grinding wheel toward the surface being ground.
Grinding arc segments in a workpiece has generally been somewhat of a trial and error process. One way machine operators grind arc segments is to overcut the desired arc by cutting a longer arc than needed, and then strip away the straight surfaces adjoining the arc along the X and Y axes until the required dimensions are achieved. Another way arcs are ground is by a process known as "contour grinding", in which the X, Y, and/or C axes are indexed simultaneously during grinding. This technique can lead to imprecise results due to the complex geometric relationships which must be controlled when indexing a plurality of axes concurrently. A third technique which has been used for arc grinding is to grind the required straight surfaces along the X and Y axes and then stop, each time an arc is to be ground, so that manual indexing of the C-axis and grinding of the arc can be achieved.
It would be advantageous to provide a jig grinder which is automated to grind arcs and straight surfaces at a constant feedrate without interruption. By reducing the number of manual operations required of the machine operator, such an automated machine tool would increase machine productivity and improve the accuracy to which parts are machined. The present invention relates to an automated machine tool in which these and other advantages are achieved.